A load bank is a device that applies a controlled, measurable electrical load to a power source so its performance can be tested, commissioned or maintained without relying on the real building load. The main types of load banks are AC resistive, resistive plus reactive (combined), DC, liquid-cooled, data-centre and server-emulator units, and the right choice depends on what you are testing (an engine, an alternator, a battery string or a cooling system) and at what power factor.
Key takeaways
- A load bank applies a controlled, measurable load so power sources can be proven during commissioning, acceptance testing and maintenance, and to prevent generator wet stacking.
- The six main types of load banks are AC resistive, resistive + reactive, DC, liquid-cooled, data-centre and server-emulator units.
- Resistive loads (kW, unity power factor) test the engine and cooling; reactive loads (kVAR) test the alternator and voltage regulator; a resistive-only load reaches only about 80% of nameplate kVA.
- Combined resistive + reactive units at 0.8 power factor are needed to prove a generator to 100% of nameplate kVA, adding roughly 75 kVAR per 100 kW.
- DC load banks test batteries, UPS and telecom power by voltage and current (commonly 12 to 480 VDC), not by power factor.
- Liquid-cooled, data-centre and server-emulator load banks validate the power and cooling infrastructure of high-density and AI data centres, where racks can draw 40 to 100 kW.
- Choose by what you are testing, at what power factor, and at what capacity, duration and standards (IEC, UL, CE, UKCA and regional marks; 50 or 60 Hz).
What is a load bank and what does it do?
A load bank is a self-contained device that creates an artificial electrical load, applies it to a power source, and dissipates the resulting energy, usually as heat. It lets engineers load a generator, UPS, battery bank or electrical supply to a known level, on demand, without needing the site's real load to be present or at risk.
The core purpose is verification. A power source is only proven when it has carried a real load. Load banks let you confirm that a standby generator will deliver its rated output, that a UPS will support its design load through a mains failure, that a battery string holds its rated capacity, and that cooling and power-distribution systems behave correctly at full duty. This is essential during commissioning (proving a new installation), acceptance testing (proving equipment meets its contract rating) and routine maintenance (proving equipment still performs over its life).
A second purpose is engine and system health. Diesel generators that run for long periods lightly loaded suffer wet stacking, where unburned fuel and carbon accumulate in the exhaust, turbocharger and combustion chambers, degrading efficiency and reliability. Running a generator against a load bank raises exhaust temperatures enough to burn off these deposits and restore the set to healthy operation.
Every load bank shares three functional parts: the load elements themselves (resistors, reactors or electronic circuits), a control system that switches load in defined steps and holds the target level, and a means of cooling, typically forced-air fans or a liquid circuit that carries heat away.
What are the main types of load banks?
Load banks are grouped by the kind of load they present and by how they are cooled and packaged. The main types of load banks are: AC resistive, resistive plus reactive (combined), DC, liquid-cooled, data-centre and server-emulator units. Each targets a different part of a power system.
The single most important distinction is resistive versus reactive. A resistive load draws real power, measured in kilowatts (kW), at unity power factor (1.0), and tests everything that produces power, principally the engine, its fuel system and its cooling. A reactive load draws reactive power, measured in kilovolt-amperes reactive (kVAR), and tests the alternator, exciter and voltage regulator, the parts that manage voltage rather than raw power. Most real-world electrical loads are a mixture of the two, which is why combined units exist.
- AC resistive: real power (kW) at unity power factor; tests the engine, fuel and cooling systems.
- Resistive + reactive (combined): kW plus kVAR at a defined power factor (commonly 0.8 lagging); tests the full generator set to its nameplate kVA.
- DC: applies a controlled current to batteries and DC supplies; tests capacity and discharge performance.
- Liquid-cooled: dissipates heat into a liquid circuit instead of air; suits high power density and low-noise or indoor use.
- Data-centre: air- or liquid-cooled units built for UPS, PDU and facility commissioning at megawatt scale.
- Server-emulator: rack-format units that mimic the electrical and thermal footprint of IT servers, testing cooling and power distribution before live hardware is installed.
AC resistive load banks: what are they used for?
An AC resistive load bank is the most common and most fundamental type. It uses banks of resistive elements to convert electrical energy directly into heat, presenting a pure real-power load at unity power factor (1.0) and dissipating that heat with forced-air fans.
Because a resistive load draws real power (kW), it fully exercises the prime mover of a generator set, its diesel or gas engine, along with the fuel, cooling and exhaust systems. This makes resistive load banks the standard tool for routine generator exercising, wet-stacking prevention and annual compliance testing.
One key limitation to understand for sizing: a resistive load bank can fully load a generator to 100% of its nameplate kW rating, but because generators are rated in kVA at a power factor (typically 0.8), a purely resistive load only reaches about 80% of the nameplate kVA. It loads the engine fully but does not fully load the alternator. For most maintenance and monthly testing this is exactly what is required; for full nameplate proving you need a reactive element as well.
Typical ratings range from a few kilowatts for portable units up to several megawatts for large containerised systems, at standard mains voltages and both 50 Hz and 60 Hz. Portable and trailer-mounted resistive units are widely used for field testing, while large fixed units support data centres and power stations.
Resistive + reactive (combined) load banks: when do you need reactive load?
A resistive plus reactive load bank combines resistive elements with inductive reactors (and sometimes capacitive elements) so it can present a load at a controlled power factor rather than unity. The standard target is 0.8 lagging power factor, which mirrors how most commercial and industrial facilities actually load a generator: roughly 80% real power (kW) and 20% reactive power (kVAR).
The reason to add reactive load is that it tests parts a resistive load cannot reach. Reactive current loads the alternator, exciter and automatic voltage regulator (AVR), verifying that the machine holds voltage and frequency when a realistic, partly inductive load is applied and when large load steps are switched. This is the only way to prove a generator to 100% of its nameplate kVA and to confirm real-world transient behaviour.
The sizing convention is straightforward: for each 100 kW of resistive load, about 75 kVAR of inductive load is added, so that the combined load sits at 0.8 power factor. For example, a 500 kW resistive plus 400 kVAR inductive combination gives roughly 625 kVA at 0.8 PF.
Combined load banks are the tool of choice for factory acceptance testing (FAT), critical-facility commissioning and any situation where the alternator and controls, not just the engine, must be proven. For a standard monthly or annual compliance test on an installed set, a resistive-only unit is usually sufficient; reactive capability is specified when full-rating or high-reliability proof is needed.
DC load banks: testing batteries, UPS and telecom power
A DC load bank applies a controlled direct-current load to a DC source rather than an AC one. Instead of testing an engine or alternator, it tests stored and DC-supplied energy: battery strings, UPS DC buses, telecom power plants, DC generators and solar or other DC systems.
The principal use is battery capacity and discharge testing. By drawing a defined, steady current (constant-current control is common) and recording voltage over time, a DC load bank measures how much capacity a battery string actually delivers against its rated ampere-hours, and reveals weak or failing cells before they cause an outage. Better units log the voltage of each individual cell throughout the discharge.
DC load banks are specified by voltage and current windows rather than by power factor. Common nominal DC voltages include 12, 24, 48, 125, 240 and 480 VDC, covering the 48 V and 125 V systems typical of telecom and utility substations and the higher-voltage buses found in large UPS installations. Current capability commonly extends from a few amps to well over a thousand amps depending on the model.
Typical users are telecom operators, electrical utilities, UPS owners and data centres, anywhere a battery is the last line of defence and its true capacity must be known, not assumed.
Liquid-cooled, data-centre and server-emulator load banks
These three types share a purpose: proving the power and cooling infrastructure of large, high-density facilities, above all data centres, before and during live operation. They differ mainly in how heat is removed and how the load is packaged.
Liquid-cooled load banks reject their heat into a liquid circuit instead of blowing hot air into the room. This lets them handle very high power density in a small footprint, run far more quietly (often below 70 dB) and be used indoors without overwhelming the room's air cooling. For AI and high-performance computing halls, where a single rack can draw 40 to 100 kW, liquid-cooled rack-level units can reproduce conditions that air-cooled equipment simply cannot.
Data-centre load banks are units, air-cooled, liquid-cooled or both, engineered specifically for facility commissioning at scale. They are used to prove UPS systems, power distribution units (PDUs), generators and the HVAC or liquid-cooling plant together, so that the whole electrical and thermal chain is validated before IT equipment is installed. Many facilities deploy a mix of liquid-cooled and air-cooled units to build the most complete and realistic test environment.
Server-emulator load banks take this further by mimicking the exact electrical and thermal footprint of the IT hardware that will eventually occupy the racks. Built in server or rack form factors, they apply a resistive IT load and generate representative heat, allowing airflow, cooling and power distribution to be tuned and proven before any real servers arrive. In liquid-cooled halls, rack emulators also verify the cooling distribution unit (CDU), the piping and leak-free operation of the whole liquid loop.
How do you choose and size the right load bank?
Choosing a load bank comes down to three questions: what are you testing, at what power factor, and at what capacity and duration. Answering them in order leads you to the correct type and rating.
First, identify the source and the goal. Testing a diesel engine, preventing wet stacking or running a monthly compliance test points to an AC resistive unit. Proving a generator to full nameplate kVA, or validating the alternator and voltage regulator under realistic conditions, calls for a resistive plus reactive unit. Testing batteries, a UPS DC bus or telecom power calls for a DC unit. Commissioning a data-centre's power and cooling calls for data-centre, liquid-cooled or server-emulator units.
Second, size the capacity to the equipment. As a rule of thumb, a resistive load bank should be rated to at least 80% of the generator's nameplate kW for a standard test, and to 100% of nameplate kW to fully load the engine. To prove full nameplate kVA you combine resistive and reactive load at the equipment's rated power factor (commonly 0.8), typically adding around 75 kVAR of inductive load for every 100 kW of resistive load. Always match voltage, frequency (50 or 60 Hz) and, for DC units, the nominal bus voltage.
Third, plan for duration and standards. Compliance regimes such as NFPA 110 call for standby generators to be exercised monthly at a minimum of 30% of nameplate kW, with a stepped annual load-bank test; high-tier data-centre commissioning can require continuous full-load runs of four hours or more. Confirm that the unit and its cooling are rated for the test duration, and that it is built and certified to the relevant standards for your market (for example IEC, UL, CE or UKCA, plus regional marks).
As an international load bank manufacturer, Ashford Energy designs and supplies the full range covered here, AC resistive, resistive plus reactive, DC, liquid-cooled, data-centre and server-emulator units, built to the applicable standards for each market and sized to the application.